I love salt. It’s just delicious. I wrote this post while noshing on deliciously salty popcorn, after a dinner which I put salt on. I crave salt so much that my parents used to joke about getting me a salt lick.
And I’m not alone. Sodium is an incredibly important part of life, which means it’s also an important part of what we eat. To make sure we get enough salt, animals have evolved salt-sensing systems, and low levels (below 100 mM of NaCl) of salt are very attractive.
But there IS such a thing as too much salt. High levels of salt (>300 mM NaCl) are really aversive (from personal experience, I wonder if Carrabba’s restaurant has concentrations of salt in their food over 300 mM). Most animals will quickly turn up their noses at a high salt concentration.
You probably know that you have classes of receptors on your tongue for taste (though they are not clustered into areas of your mouth, like front for sweetness, as previously thought). You have sweet, umami (savory), bitter, sour, and salt. In most animals, sweet and umami are always attractive, while bitter and sour are nasty (except where we have overcome the aversion to enjoy things like coffee and beer). Salt, though, is the only one that goes two ways, with low levels being attractive and high levels being aversive.
Now we know how low salt works. The salt receptors that are currently known are good for detecting low salt. But high salt, that’s more difficult. First of all, our aversion to high salt concentrations is not very selective. While low salt detection is limited to good old NaCl, high salt detection is non-specific, working for many salts including NaCl, but others as well (like KCl).
Not only that, but if you block the low salt pathway (you can block the sodium channels involved by using a diuretic), the high salt pathway still functions, which means that there are other receptors involved. But what other receptors?
Well, it turns out that high salt is not just…salty. It’s BITTER. and SOUR. Or at least, your receptors think so.
Oka et al. “High salt recruits aversive taste pathways” Nature, 2013.